In electrostatographic imaging processes, such as dry electrophotographic copying, a pattern or image formed by electrostatically charged thermoplastic particles of toner powder is transferred from the surface of a photoconductor or other dielectric surface to a receiver material which can be in the form of sheets or a continuous web roll. The transfer is normally accomplished by electrically charging the receiver surface to a polarity opposite to that of the toner particles and then contacting the receiver with the photoconductive surface. After transfer of the toner particles, the receiver is passed through heated rollers to fuse the toner to its surface. A similar transfer and fusing of toner to a receiver occurs in laser printing.
Commonly, the receiver for dry toner particles is plain paper. Many thermoplastic toner materials adhere well to paper and form a satisfactory image or printing. When it is desired, however, to form a toner image on a plastic film, for example, in making a transparency for overhead projection, problems arise. One problem is the difficulty of adhesion of the usual toner particles to the kinds of films that are preferred for transparency printing. A particularly preferred type of transparent film for toner printing is a polyester film such as a film of biaxially oriented poly(ethylene terephthalate). Although, this kind of film has desirable physical properties such as thermal stability and can withstand the high temperatures encountered in electrophotographic copying machines, the polyester surface does not adhere well to the usual thermoplastic toner powders.
To improve toner adhesion to plastic receivers, the prior art has applied various coatings to their surfaces. In some instances these coatings may have improved the adhesion of toner to the receiver, but other problems have occurred. For example, in automatic copying machines, coated plastic sheets can be difficult to feed and transport rapidly and, when stacked in packages or in feeding trays and equilibrated to machine environment, the sheets often block or stick together. This results in multifeeds and jams. Especially in high temperature copiers, coated film sheets have caused serious jamming, with consequent delays in the copying operations. The prior art discloses toner receiving films having surface coatings that provide certain properties. For example, Hart, U.S. Pat. No. 5,130,189 discloses an imagable copy film comprising a biaxially oriented polyester substrate and an acrylic receiving layer. The latter can contain silica filler particles of small size i.e., less than 0.5 .mu.m in a concentration of at least 5%. The patent to Sun, U.S. Pat. No. 5,104,721 discloses an electrophotographic printing medium comprising a polymeric substrate coated with a layer of a certain hardness and Tg and containing a pigment which provides a relatively high coefficient of friction. The patent to Carls, U.S. Pat. No. 5,208,093 discloses an electrographic article for color imaging comprising a polymeric film and a receptor layer formed of a thermoplastic resin such as polyester resins, styrene resins, polymethylmethacrylate resins, etc., but especially bisphenol A polyester of 0.5 to 10 .mu.m thickness. The receptor is said to have an equivalent or lower storage elasticity modulus than the toner resin used for imaging. Certain polymeric, silica or starch particles of 5 to 25 .mu.m diameter can be added to reduce pooling of fuser oil on transparencies.
Prior art polymeric toner receiving materials, however, continue to present problems and lack the properties most desired for toner imaging with electrophotographic copying machines. In particular, they lack the combination of properties needed for high quality imaging with electrophotographic copying machines having high speed duplex feeders and high temperature fusing stations. Pigmented toner-receiving layers of the prior art, for example, exhibit opacity or haze and high coefficient of friction.
A need, therefore, exists for an improved toner receiver material, particularly in plastic sheet form, of excellent clarity which can receive thermoplastic toner particles with good adhesion and good image quality and can feed reliably in copying machines, including high speed duplex copiers and laser printers by good engagement with feeding rolls, without blocking when stacked in feed trays and without sticking to machine parts and with good release from fuser rolls, especially in high volume applications. In accordance with the present invention such an improved toner receiver material and a method for its manufacture are provided.